cause of P0300 DTC on 1996 chevy k1500 5.7L SFI?

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SES light comes on and i have gotten this code a couple times now and i would like to finally figure out whats going on. The code seems to pop up during highway driving and not necessarily under heavy load. It seems to idle a bit rough at a stop but if you mat it or are just coasting it seems to run just fine. The truck has a new motor with less than 4000 miles on it. which of course included new wires, plugs, etc. I also only fill up with shell gas which is supposedly really good. I doubt its an ignition problem but im open to suggestions i know that those SFI spider assemblies can go bad but i want to know for sure before i replace a 300$ part. anything else you need to know feel free to ask. thanks.
Forgot to mention i know the code is for "random multiple cylinder misfire" i have my own scan tool (actron CP9175) which can freeze frame data of an OTC so is there any other info i could get from that data to help with a diagnosis?
Ey thanks for the info but i meant that use shell gas just because of the quality not that theres necessarily a leak anywhere. and also could it really be any of the internals the new motor has less than 4000 miles on it? I also figured it wasnt just as simple as wires, cap, rotor and plugs i was just stating that those were replaced as well
 
Based on what you've told me about your P0300 Random Misfire problem, this appears to be beyond simply replacing plugs, wires, and coils. Typically those are the first things to check. Other things to check normally include the fuel injectors themselves. The fact that gas seems to be going into the engine somehow is definitely concerning. If gas is getting into the crankcase then your O2 sensors should not be your next concern. There are quite a few things to check - compression, valves, intake manifold gasket, etc. and if you aren't up to the task it might be better off to take it to an experienced, trusted technician for a proper diagnosis, even if you perform the repair yourself. If you want to continue diagnosis, here are some things to read first - P0300 @ OBD-Codes.com; P0300 @ Engine-Light-Help.com; P0300 @ HotRodLane; P0300 at RandomMisfire;
 
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